Ultrasound imaging systems, such as medical ultrasound systems, typically use a transducer comprised of a phased array of individually driven elements to generate interrogation signals. The most popular method, until recently, for driving ultrasonic transducer elements has been to apply timed electrical pulses to each element of the transducer. By properly adjusting the start time of the pulse for each transducer element, acoustic beams, that can be focused and steered, are formed. Images are created from the returned echoes of the acoustic beams as they are progressively swept across a target area.
Known pulse driving circuits for phased arrays transducer are described by Hans W. Persson in xe2x80x9cElectric Excitation of Ultrasound Transducers for Short Pulse Generation,xe2x80x9d Ultrasound in Med. and Bio., Vol. 7, 1981. Such drive circuits are typically limited to generating rectangular waveforms that often exhibit exponentially decaying segments. The amplitude of drive signals created by pulse generator circuits is determined by the magnitude of a programmable d.c. voltage source. Programmable d.c. voltage source typically do not have the agility to change rapidly between transmit bursts, limiting the ability to create transmission signals with different amplitudes on alternate transmit bursts, a capability that is highly desirable for mixed mode operation.
Pulse generator circuits are often configured to provide signals of different magnitudes (amplitudes) to the individual elements of a transducer array. The arrangement of signal amplitudes applied to the elements of transducer arrays are often referred to as amplitude apodization profiles. Proper application of amplitude apodization reduces the magnitude of side lobes in transmitted acoustic beams. Apodization profiles are typically created using banks of pulse generating circuits powered by independent programmable d.c. voltage sources. The number of apodization levels is limited to the number of programmable d.c. voltage sources. For practical reasons, only a small number of programmable d.c. voltage sources can be provided, resulting in a piecewise approximation of the intended apodization profile.
The deficiencies of pulse generator circuits are most apparent in imaging modalities where a signal is transmitted at a fundamental frequency and images are constructed from received harmonic signals generated by non-linear acoustic propagation, so-called xe2x80x9charmonic imagingxe2x80x9d. Similarly, pulse generator circuits exhibit less than satisfactory results when used to image harmonic signals generated by contrast agents. The basic reason for these unsatisfactory results is that the harmonic content of signals produced by pulse generator circuits typically exceeds levels required for optimal harmonic imaging modalities. In such imaging modalities, the harmonic content of the transmitted signal effectively increases the noise floor of the received harmonic signal. For optimal performance transmitted harmonics must be suppressed.
So-called arbitrary waveform generators have been advanced as a solution to the above noted problems with pulse driven phased array transducers. Arbitrary waveform generators use stored digital representations of shaped waveforms to generate, using a digital-to-analog converter, an analog drive signal for the transducer. The drive signals produced by an arbitrary waveform generator are typically gaussian or hamming modulated cosines individually formed for each transducer element. Arbitrary waveform generators can provide instantaneous change in transmit energy between transmit pulses, apodization profiles with greater resolution, and acoustic beams with lower harmonic content.
A basic arbitrary waveform generator, used to drive ultrasonic transducer elements, is described by R. Y. Liu in xe2x80x9cThe Design of Electric Excitations for the Formation of Desired Temporal Responses of Highly Efficient Transducers,xe2x80x9d Acoustical Imaging, Vol., 12, 1982. The system described by Liu uses memory to store digitized waveform samples, a functional module to retrieve samples from memory, a digital to analog converter, and a broadband driver to excite a transducer element. In Liu""s system, digitized waveform samples are pre-calculated and stored in memory.
John A. Hossack, et al., discloses the use of an arbitrary function generator to generate an excitation signal for an ultrasound transducer in xe2x80x9cImproving the Characteristics of a Transducer Using Multiple Piezoelectric Layers,xe2x80x9d IEEE Transactions on Ultrasonics, Ferroelectrics, and Frequency Control, Vol. 40, No. 2, March 1993. The Hossack et al. ultrasonic system includes a computer that stores a digital waveform in a memory integrated circuit chip. A digital counter sequentially addresses the memory integrated circuit and data is read from the memory when addressed. Data read from the memory integrated circuit is converted by a digital-to-analog converter (DAC) and amplified to drive an ultrasound transducer.
U.S. Pat. No. 5,675,554 (Christopher R. Cole et al.) describes an arbitrary waveform generator used to drive individual elements of an ultrasonic transducer array in medical imaging applications. This implementation utilizes a digital memory to store a pre-calculated time domain transmit waveform envelope for each transmit channel. These transmit waveform envelopes are base band (near zero Hz) and may be of arbitrary shapes including variations of Gaussian and Hamming. Upon the start of a transmit event and after an appropriate focusing delay, each channel""s transmit waveform envelope is retrieved from memory and sent through a dedicated signal processing path where apodization weighting and fine focus delay adjustments are applied by digital multipliers. An amplitude modulator in each digital signal processing path modulates a high frequency carrier with the channel""s base band waveform envelope. The resulting digital version of the amplitude modulated transmit signal is converted by a digital to analog converter and amplified prior to driving an element of an ultrasonic transducer array.
U.S. Pat. No. 5,608,690 (Hossack et al.) and U.S. Pat. No. 5,740,128 (Hossack et al.) disclose an arbitrary waveform generator for driving individual elements of a transducer array. The described arbitrary waveform generator operates similar to that described in the earlier mentioned academic paper published by Hossack et al., xe2x80x9cImproving the Characteristics of a Transducer Using Piezoelectric Layers.xe2x80x9d This architecture utilizes digital memory to store the actual time domain waveform samples that are sent to analog to digital converters and amplifiers so as to drive individual elements of transducer arrays. The waveforms stored in memory are digital versions of the envelope modulated drive signals sent to each ultrasound transducer element.
The arbitrary waveform generators described above have limitations. The amplitude of transmit signals generated by these implementations are controlled by adjusting the magnitude of the stored digital representation of the waveform. This requires the re-calculation and the re-storing of the waveform for each change in amplitude. Not only is this approach resource intensive, but it also results in the undesirable reduction of transmit signal resolution. To put it another way, as the amplitude of the signal being described decreases, the ability to accurately describe that signal also decreases since fewer digital bits are used to represent the signal.
Arbitrary waveform generators that adjust acoustic power by scaling the digital representation of transmit signals also increase harmonic content as acoustic power is reduced. This is because, as transmit signal resolution is reduced, the harmonic content of the transmit signal increases. While, the harmonic content can be removed from the transmit signal with high order low pass filters, such a solution is unwieldy due to the large bank of filters required for each transmit channel.
An arbitrary waveform generator that stores an optimized digital representation of a waveform in a memory and, after retrieval thereof, adjusts an amplitude of the waveform to generate an analog signal for exciting an element of an ultrasonic transducer. The arbitrary waveform generator includes an arithmetic element that accesses digitized waveform samples (or xe2x80x9cdigital waveform samplesxe2x80x9d) from a waveform sample memory and adjusts the amplitude thereof. Preferably, the arithmetic element is a multiplying digital-to-analog converter (MDAC) that has a first input connection for receiving digitized waveform samples and has a second input connection for receiving a reference signal. The output signal from the MDAC is a mathematical product of the digitized waveform samples and the reference signal. A controller provides the reference signal based on a requested power level and/or an imaging modality being utilized. The optimized digital representation of the waveform may be updated as needed, for example by the controller.